1. Field of the Invention
The present invention relates to a reducer, and especially relates to a reducer which is provided with a planetary gear speed reduction mechanism. The planetary gear speed reduction mechanism, having an internal gear, an external gear which internally meshes with the internal gear, and a swing body which makes one of the internal and external gears oscillatingly rotate on its axis in accordance with a rotation of an input shaft.
2. Description of the Related Art
When a certain speed reduction ratio is required with respect to rotational speed of a motor, doubly or triply stacking speed reduction stages with a low speed reduction ratio causes lack of miniaturization. A speed reducer having a planetary gear speed reduction mechanism is known as a conventional speed reduction mechanism which can obtain a high speed reduction ratio by a single stage. In the planetary gear speed reduction mechanism, an external gear internally meshes with an internal gear. The speed reducer with the planetary gear speed reduction mechanism is used in many cases or in many apparatuses.
FIG. 8 shows an example of this type of planetary gear speed reducer, which is disclosed in Japanese Patent Laid-Open
This planetary gear speed reducer G is provided with an input shaft (a first shaft) 1, an eccentric body (swing body) 3, an external gear 5, an internal gear 10, and an output shaft 2. Motive power is inputted from a motor 20 into the input shaft 1. The eccentric body 3 is rotated by a rotation of the input shaft 1. The external gear 5 is installed to oscillatingly rotate with respect to the input shaft 1 through the eccentric body 3. The external gear 5 internally meshes with the internal gear 10. The output shaft 2 is coupled to the external gear 5 in such a manner as to transmit only a rotational component thereof. The external gear 5, as shown in FIG. 9, is fitted on the eccentric body 3 with play by a bearing hole 5a formed in the center of itself. An inner ring 3a (of the bearing) is provided on the outer periphery of the eccentric body 3. Bearing rollers 4 are directly inserted between the inner ring 3a and the bearing hole 5a. Furthermore, a plurality of inner roller holes 6 are formed in the external gear 5 in a circumferential direction. An inner pin 7 and an inner roller 8 are fitted into each of the inner roller holes 6 with play. External teeth 9 in the shape of a trochoid or an arc, which are formed in the outer periphery of the external gear 5, are engaged with the inside of the internal gear 10.
The inner roller 8 is rotatably held by the inner pin 7. The base portion of the inner pin 7 is fixedly fitted into a carrier 14 which is integrated with the output shaft 2. The internal gear 10 also serves as a part of a casing 12, and internal teeth of the internal gear 10 are structured by outer pins 11. The output shaft 1 is firmly supported by two large bearings 16 and 18 so that the center of the shaft is not misaligned.
When the input shaft 1 performs a single rotation, the eccentric body 3 performs a single rotation in accordance therewith. The external gear 5 also tries to rotate in accordance with the rotation of the eccentric body 3, but engagement with the internal gear 10 restricts free rotation of the external gear 5 on its axis. Thus, the external gear 5 almost only oscillates with engaging with the internal gear 10 (with slight rotation on its axis).
Taking a case that the number of the teeth of the external gear 5 is represented by N, and the number of the teeth of the internal gear 10 is represented by N+1, difference between the numbers of the teeth is 1. Therefore, whenever the input shaft 1 performs a single rotation (in other words, the external gear 5 oscillates one time), the external gear 5 deviates with respect to the internal gear 10 by a single tooth (slightly rotates on its axis). This means that the external gear 5 rotates on its axis at a speed of −1/N times the speed of the rotation of the first shaft 1 (a minus represents reverse rotation).
Of movement of the external gear 5 like this (oscillation with slow rotation on its axis), an oscillating component is absorbed by clearance between the inner roller holes 6 and the inner rollers 8, and only a rotational component is transmitted to the output shaft 2. As a result, speed reduction at a speed reduction ratio of −1/N is achieved between the input shaft 1 and the output shaft 2.
As other speed reduction mechanisms in which an external gear internally meshes with an internal gear, are known an inner gearing planetary gear speed reduction mechanism of an internal gear oscillation type in which an internal gear oscillatingly rotates about a fixed external gear, a planetary gear mechanism of a distortion engagement type in which a deformable external gear is inscribed in an internal gear with flexure, and the like.
FIG. 10A is a sectional view of a conventional planetary gear speed reducer of a distortion engagement type, which is disclosed in Japanese Patent Laid-Open Publication No. 11-193852. FIG. 10B is a side view of a wave generator in the planetary gear speed reducer. This planetary gear speed reducer G2 of the distortion engagement type has a ring-shaped rigid internal gear 22, a cup-shaped flexible external gear 23 disposed inside the rigid internal gear 22, and a wave generator (swing body) 24 fitted into the inside of the distortable external gear 23. An outline of the wave generator 24 takes the shape of an ellipse. Internal teeth 22a are formed in the inner periphery of the rigid internal gear 22. The cup-shaped flexible external gear 23 is provided with a cylindrical barrel 23a, a ring-shaped diaphragm 23b closing one end of the barrel 23a, a ring-shaped boss 23c continued from an inner peripheral edge of the diaphragm 23b, and external teeth 23d formed in the outer periphery of an open end of the barrel 23a. The external teeth 23d are engageable with the internal teeth 22a. The number of the external teeth 23d is generally less than that of the internal teeth 22a by two.
The wave generator 24 elliptically distorts the flexible external gear 23, so that the external teeth 23d of the flexible external gear 23 are engaged with the internal teeth 22a of the rigid internal gear 22 at two points. Since engagement portions are moved in a circumferential direction, relative rotation occurs between the flexible external gear 23 and the rigid internal gear 22 in accordance with difference between the number of the external teeth 23d and the number of the internal teeth 22a. Generally, an input shaft (the first shaft: not illustrated) is coupled to the wave generator 24, and the rigid internal gear 22 is fixed on the wave generator 24. An output shaft (not illustrated) supported by a bearing (not illustrated) is coupled to the boss 23c of the flexible external gear 23. Thus, it is possible to take out rotation with reduced speed via the output shaft supported by the bearing (refer to, for example, Japanese Patent Laid-Open Publication No. 7-119800 and other articles). The principle of speed reduction is basically the same as that of the foregoing inner gearing planetary gear mechanism.
The reducers having these planetary gear speed reduction mechanisms have the common advantage of obtaining a large speed reduction ratio in a single stage. Any of the reducers, however, needs an additional mechanism for absorbing an oscillating component or a distortion component, because the external gear or the internal gear rotates (rotates on its axis) with oscillation or distortion. Also, the output shaft of the reducer is supported by the bearing in such a manner as not to deviate the center of the shaft, in order to take out only the rotational component.
To absorb the oscillating component in the inner gearing planetary gear speed reduction mechanism, as described above, a method of fitting the inner pins into the inner pin holes with play is adopted in general. In addition, when an Oldham coupling is provided or there is spatial room in an axial direction, a structure of combination of the so-called dog bone and a universal joint may be adopted.
In the case of the planetary gear speed reduction mechanism of the distortion engagement type, as has been already described too, a method of extending the external gear in an axial direction in a tubular shape and absorbing the distortion component by an extended portion is often used. Therefore, the external gear of the distortion engagement type planetary gear mechanism is generally made of a material which has flexibility and high strength.
The reducers having these planetary gear speed reduction mechanisms, however, have the following problems.
First, there is a problem that the mechanism for absorbing the oscillating component or the distortion component requires extremely high accuracy in processing or preparation of expensive material, so that manufacturing cost of the whole device becomes high.
Second, the oscillating component or the distortion component is absorbed along the axial direction of the input shaft (the first shaft), and motive power is outputted from the output shaft coaxial with the input shaft, so that the axial length of the device tends to be long.
Third, obtaining the high speed reduction ratio in the single stage is a big advantage, but, in other words, this means that the heavy output shaft has to be supported with accurately maintaining coaxiality with the first shaft. Thus, the cost for supporting the output shaft and its peripherals, including manufacturing and assembling cost of the bearing and the like, tends to be high, and hence it becomes a large cause of increasing cost of the whole drive device.
Fourth, since the whole drive device becomes heavy in weight, the drive device is inconvenient to handle. Also it is necessary to secure strength in structure of a host machine to support the heavy drive device.